Plant Resistance to Plant Pathogens
Plants have evolved highly effective mechanisms for resistance to disease caused by infectious agents, such as bacteria, fungi and viruses. This resistance can be caused by several mechanisms, the best known of which are the systemic acquired resistance (SAR; Ross, 1961; Durrant and Dong, 2004) and induced systemic resistance (ISR; van Loon et al., 1998). In the most simple case, the inducer is the plant pathogen itself, in other cases, the inducer can be either a chemical compound (salicylic acid, benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester also known as BTH) or physical impact such as water or heat stress (Walters et al., 2005). It appears that induced systemic resistance depends upon a gradual expression and persistence of a low level of metabolic perturbation. Unlike elicitors of phytoalexin accumulation, which elicit at the site of application and may be responsible for localized protection, inducers of systemic resistance sensitize the plant as a whole to respond rapidly after infections. These responses include phytoalexin accumulation, lignification and enhanced activities of chitinase and glucanase.
Extract from giant knotweed (Reynoutria sachalinensis) sold as MILSANA® and REGALIA® by Marrone Bio Innovations, Inc.) provides control of powdery mildew and other plant diseases on cucurbits and other crops mainly by inducing an accumulation of fungitoxic phenolic compounds in the plant (Daayf et al., 1995; Wurms et al. 1999; Schmitt, 2002). Recently, formulated giant knotweed extract has also shown great efficiency in inducing resistance in various crops and plant pathogens including wheat powdery mildew (Vechet et al., 2009). Besides the ISR mode of action, the formulated R. sachalinensis extract has recently also been shown to have a direct fungistatic effect against wheat powdery mildew (Blumeria graminis f. sp. tritici; Randoux et al., 2008).
Fungicide Resistance
Fungicide resistance is a common phenomenon in pests including plant pathogens. When a fungicide, especially those with single-site mode of action, is frequently used, the targeted pathogen can adapt to the fungicide due to high selection pressure. It is estimated that pests can develop resistance to pesticides within 5-50 generations (May, 1985). Most plant pathogens fit in this range in one growth season and thus can develop fungicide resistance quickly. For example, it only took one year for benomyl lost efficacy for control of cucurbit powdery mildew after its first registration for commercial use (McGrath, 2001).
Quinone outside inhibitors (also known as QoI fungicides or strobilurins) has been widely used to control agriculturally important fungal pathogens since their introduction in 1996. Strobilurins block the respiration pathway by inhibiting the cytochrome bc1 complex in mitochondria, thereby blocking the electron transfer process in the respiration chain and causing an energy deficiency due to lack of adenosine triphosphate (ATP) (Bartlett et al., 2002). Strobilurins and other fungicides with a single-site mode of action such as demethylation inhibitors (DMI) are prone to resistance development among plant pathogens. To date, several plant pathogenic fungi have developed field resistance to strobilurins (Tuttle McGrath, 2003; Fraaije et al., 2003) and DMI fungicides (Schnabel et al., 2004), and considerable effort has been made worldwide to develop appropriate resistance management strategies with detailed recommendations of how to combine fungicides and other antifungal compounds in programs and rotations to minimize the risk of resistance development (Tuttle McGrath, 2006; Wyenandt et al., 2009).
Methods to Control Fungicide Resistance
The most common strategy to manage fungicide resistance is to use site-specific fungicides that are prone to resistance development in a combination (pre-mix or tank mix). Besides resistance management, tank mixes also offer a compensatory mechanism in case of a failure of one fungicide as well as a way to reduce the dose to reduce selection pressure on pathogens (van den Bosch and Gilligan, 2008). In some cases, the combination of single and multisite fungicides in a tank mix or in rotation can provide additive or even synergistic interactions (Gisi, 1996). Holb and Schnabel (2008) were able to show improved control of brown rot (Monilinia fructicola) in a field study with a tank mix of a DMI fungicide and elemental sulfur, and Reuveni (2001) demonstrated the benefits of using strobilurins and polyoxin B fungicides in combination with sulfur to control powdery mildew in nectarines.
Plant defense inducers such as the extract of R. sachalinensis have been tested in tank mixes and rotations with other SAR/ISR products as well as with biocontrol agents (BCA) (Hafez et al., 1999; Belanger and Benyagoub, 1997; Schmitt et al., 2002; Schmitt and Seddon, 2005; Bardin et al., 2008). The purpose of these studies has mainly been to demonstrate the compatibility of different types of plant extracts with biocontrol agents. Konstatinidou-Doltsinis et al. (2007) tested the R. sachalinensis product in a rotation with Pseudozyma flocculosa product against powdery mildew on grapes, and found that alternated application of both products improved the efficacy of R. sachalinensis. In the same study, alternation of sulfur and R. sachalinensis in a rotation did not have a beneficial effect. Belanger and Benyagoub (1997) found that a yeast-like fungus, Pseudozyma flocculosa, was compatible with R. sachalinensis when used against cucumber powdery mildew in a greenhouse. Similarly, Bokshi et al. (2008) evaluated the combined effect of an acquired systemic resistance activator benzothiadiazole and MILSANA® against cucumber powdery mildew, and found that MILSANA® used in a rotation with benzothiadiazole provided an effective control measure against powdery mildew in the field. However, based on the disease severity and yield data collected, it was not possible to determine whether the positive effect was additive or synergistic.
Pesticide synergism has been defined as “the simultaneous action of two or more compounds in which the total response of an organism to the pesticide combination is greater than the sum of the individual components” (Nash, 1981). Hence, when fungicides interact synergistically, a high level of disease control is achieved with less than label rates of each individual fungicide. Usually, the best effect is achieved with combinations of fungicides with different modes of action (MOA), but synergy has also been demonstrated in combined use of products with similar mode of action (De Waard, 1996). Fungicide synergism has been demonstrated mostly in laboratory studies (Samoucha and Cohen, 1984; Gisi, 1996) but in some cases (Karaogladinis and Karadimos, 2006; Burpee and Latin, 2008) synergism has also been found in the field studies. Additionally, synergism of antifungal compounds other than fungicides (bicarbonates and refined petroleum distillate) has been demonstrated against rose powdery mildew and black spot (Horst et al., 1992).